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Hepnntea trOm foIOWDI:.R COATING, I-ebruary '996
How to eliminate outgassing, the
powder coating faux pas
Nick Liberto, P.E.
Powder Coating Consultants
Division ofNinan, Inc.
ost etiquette experts in this
country would find the act
of expelling bodily gases in
public as quite socially unacceptable.
In some countries, however, the
same act is greeted as a compliment
to a well-prepared and tasty meal.
In the powder coating industry, out­
gassing is typically met with the
same attitude that meets a loud
belch at the dinner table in a fine
restaurant here. This article de­
scribes how and why outgassing
occurs and what steps can be taken
to reduce or eliminate its effect on
powder-coated surfaces.
M
What is outgassing?
Outgassing occurs when trapped
gases are released through a powder
coating during the cure process.
When the gas passes through the
coating, it forms pinholes. Moisture
or corrosive materials can then pen­
etrate the coating via the pinholes
and corrode the part. The pinholes
can also be unsightly on high-quali­
typarts.
What causes outgaSSing?
Many things cause outgassing on
powder-coated parts. Outgassing
can be attributed to the substrate
material, to surface contamination,
and to the powder itself. Outgassing
can occur for the following reasons:
Castings. Both sand- and die-cast
pouring processes trap gases in cast­
ings. These gases can be anything
from trapped air to gas formed when
molten metal cools. The quality of
the metal and the care taken in the
pouring process directly affect the
amount of air trapped. A casting of
high-quality metal, for instance,
usually has a low amount of trapped
gas. Outgassing can occur when a
substrate containing trapped air is
powder coated.
Galvanized (zinc-coated) sub­
strates. Outgassing can occur when
Ii powder coating is applied to a
zinc-coated substrate, galvanized
steel, for example. As with castings,
the zinc application processes
produce gases that can be trapped
withinthesubsequentsurfacecoa~
ing. This is especially true with hot­
dip galvanizing. Outgassing doesn't
occur with galvannealed steel, how­
ever, because the surface has been
annealed to relieve the stresses and
release the entrapped gases.
Surface contamination. Out­
gassing can occur when a powder
coating is applied over a substrate
with surface contaminants. The con­
taminant can be oil, grease, mold
release, or any other material that
can vaporize through the powder
coating during the curing process.
Although the problem is usually
labeled as stemming from improper
pretreatment of the substrate, not
outgassing, the defect on the coated
surface is the same.
Coating thickness. Outgassing
can also occur when powder is
applied thickly during a single pass.
In this case, gases released during
the curing process burst through the
coating surface, causing pinholes.
How can you eliminate outgassing? Now that you know what out­
gassing is and how it occurs with
some substrates, you should evalu­
ate the methods of reducing or elim­
inating the problem. In some cases,
it's unnecessary to completely elim­
inate the problem. For example, if
the outgassing occurs with a nonap­
pearance part that doesn't need
high corrosion resistance, you
shouldn't really worry too much
about it. Certainly you couldn't jus­
tify the costs of taking care of the
outgassing problem. The first ques­
tion you need to ask yourself is this:
Does this part require the corrosion
protection or high-quality appear­
ance that outgassing interferes
with? If you answered yes, then
read on for ways to take care of the
outgassing problem.
Preheating the pari. This is the
most popular method of dealing
with outgassing. The part is pre­
heated to release the trapped gas
before powder coating. You can pre­
heat the part in the dry-off oven
with a temperature higher than the
cure temperature. Or you can allow
the part to travel through the entire
powder coating line, including the
cure oven, without applying the
coating. The heat from the cure oven
drives out the trapped gas. Then,
you can coat the part on its second
trip through the line. The second
method is normally used if the plant
has the conveyor time or if the dry­
off oven temperature can't be raised
high enough. This method may not
eliminate outgassing if the part
has a lot of trapped gas. If the part
has that much trapped gas, it just
keeps releasing it no matter how
many times you preheat the part.
Aging the castings. In the past,
castings were allowed to age natu­
rally, which released trapped gas
and made the material more stable
for machining. In today's market­
place, however, waiting 6 months or
more for castings to age before they
can be used is all but unthinkable.
There are ways you can accelerate
this process by using artificial aging
techniques. Contact your casting
supplier or talk to a metallurgist for
details.
Sealing the part surface. This
method requires you to apply a
primer or another material to seal
the gas inside the substrate, which
eliminates the outgassing. Some
companies specialize in impregna­
tion processes with such materials.
Contact a primer coating manufac­
turer to find out about sealant prod­
ucts. [Editor's note: For more infor­
mation, see "Outgassing expertise
with castings fuels job shop's push
into powder," by Matthew Knopp,
Powder Coating, vol. 6, no. 5
(October 1995). Also see "A job shop
masters methods to outwit out­
gassing," Powder Cooting, vol. 4, no.
4 (August 1993).]
Changing the curing technolo­
gy. Switching to an infrared cure
oven can eliminate outgassing.
Infrared ovens heat only the part
surface, not the part. And heated
parts give off gas. This method may
not work for you, however, because
of line-of-sight problems associated
with IR curing of complex parts.
Formulating the powder. You
may have to ask your supplier to
improve the flow characteristics of
the powder coating you use.
Improving flow enables the powder
to remain in a melted form for a
longer time during the cure process,
which allows the gases to escape.
The coating flows over the pinholes
in the castings and creates a smooth
finish. However, high-flow powders
can cause problems more difficult to
solve than outgassing. A high-flow
powder may sag more easily than
other powders and may draw back
from sharp edges to reduce edge cov­
erage. You should talk to your pow­
der supplier about what features are
more important for your specific
application. You could also replace a
powder coating that appears smooth
when cured with one that appears
textured when cured. The texture
will mask the outgassing pinholes,
but it won't improve the part's corro­
sion resistance. The pinholes will
still be there to allow corrosive liq­
uids to penetrate the coating.
Changing the substrate. This
could be your best bet if you can't
use any of the other methods dis­
cussed here to eliminate outgassing.
I know it sounds like you may be
giving up, but sometimes retreating
is the best strategy.
Eliminating the contamination.
When outgassing occurs on parts
with surface contamination, the
solution is easy: Clean the part
before you powder coat it. Find the
contaminant and remove it before
coating the substrate, and the out­
gassing problem will go away.
Contact your pretreatment chemical
supplier for help.
Controlling the coating film
thickness. If excess film thickness
on the part is causing outgassing
problems, then reducing the film
thickness is the easiest way to get
rid of the outgassing. If you require
a thick film build for the parts, you
could try applying the powder in
two coats. If that's not practical,
have your powder coating supplier
reformulate the powder so that it
can be used in a thick-film-build
application.
How do you stay free of
outgassing?
You can see that outgassing in the
powder coating industry doesn't
have to raise eyebrows. It's neces­
sary to first find the cause of the out­
gassing on the part and then start
investigating solutions. Following
the recommendations in this article
should help you find a solution that
works for your substrate. And
remember, never allow those eye­
brow-raisers who say gas shouldn't
be expelled in public to intimidate
you in your search for the outgas­
free part.
PC